BACKGROUND/AIMS Although guidelines recommend a 2 cm distal margin in sphincter-saving operations for rectal cancer, some studies have shown that it may be decreased to 1cm after preoperative radiotherapy. At the present time, there are no established guidelines that suggest a specific distal safety margin for rectal cancer after preoperative radiotherapy. This study aims to examine whether preoperative radio therapy can reduce the distal safety margin in the treatment of lower rectal cancers. METHODOLOGY We examined the distal spread by H&E and immunohistochemical staining of CAM5.2 (epithelial marker) in serial sections of surgically resected specimens. To evaluate the extent of distal intramural spread, we defined the "DS length" as the distance between the microscopically defined distal tumor border and the distal spread. We compared the DS length between 20 patients who underwent preoperative radiotherapy followed by surgery (Rad (+) group) and 20 surgery-alone (Rad (-)group). RESULTS The average DS length was significantly smaller in the Rad (+) group (3.2mm) than in the Rad (-) group (6.3mm) (p=0.028). Furthermore,the greatest DS length was 5.8mm in the Rad (+) group,but 11.5mm in the Rad (-) group. No patient showed a DS length of over 1cm in the Rad (+) group. CONCLUSIONS These results suggested that the safety margin may be reduced to 1cm by preoperative radiotherapy.Therefore, preoperative radiotherapy may extend the indications for sphincter-saving operation.